1. Field of the Invention
The present invention relates to a sheet supply apparatus for supplying a sheet (for example, printing sheet, transfer sheet, photosensitive sheet, electrostatic recording sheet, print sheet, OHP sheet, envelope, post card, sheet original and the like) from a sheet stacking portion to a sheet treating portion (for example, recording portion, reading portion, working portion and the like) in an image forming apparatus such as a recording device, i.e., a printer (as an information output device of a word processor, a personal computer and the like), a copying machine, a facsimile machine and the like, and a recording apparatus having such a sheet supply apparatus.
2. Related Background Art
In sheet supply apparatuses, it is required that stacked sheets are surely separated one by one and the separated sheet is positively supplied. In the past, there has been proposed a technique in which a pawl member is provided in association with a front corner of a sheet stack so that, when sheets are fed out by a sheet supply roller, only an uppermost sheet is flexed to ride over the pawl member, thereby separating the sheets one by one. However, in this technique, sheets having great resiliency (hard to be flexed) such as envelopes, post cards and the like cannot surely be separated one by one.
On the other hand, there has been proposed a technique in which sheets hard to be flexed (for example, envelopes, post cards and the like) can be separated one by one (see Japanese Patent Appln. Laid-Open No. 3-284547). This technique will now be explained with reference to FIG. 11. A sheet stacking plate 201 for stacking sheets thereon is biased upwardly by a spring member 203. A free roller 204 for regulating an upper surface of the sheet stack is contacted with an upper surface of the sheet stack on the sheet stacking plate 201 to maintain the upper surface of the sheet stack below a guide surface 205. Further, an inclined surface 207 for separating sheets is disposed at a downstream side of the sheet stacking plate 201.
A sheet supply roller 206 is constituted by a semi-circular roller having a large diameter portion and a small diameter portion (cut-out portion) so that, when the large diameter portion of the sheet supply roller is urged against an upper surface of the sheet stack on the sheet stacking plate 201, one or several sheets are fed out. The sheets fed out by the sheet supply roller 206 abut against the inclined surface 207. In this case, an uppermost sheet rides over the inclined surface 207 while being flexed, thereby separating the uppermost sheet from the other sheet(s). Since tip ends of second and other sheets are pressed downwardly by an elastic force of the flexed uppermost sheet, these sheets cannot ride over the inclined surface 207. In this way, the sheets are positively separated one by one.
However, in such a separating mechanism, since the elastic force generated in the uppermost sheet when the uppermost sheet is flexed around a contact point P between the sheet and the free roller 204 acts on the tip ends of the second and other sheets and affects a great influence upon the separation of the sheets, the inclination of the inclined surface 207 must be set in accordance with the flexural rigidity of the sheet. That is to say, when the sheets having great flexural rigidity are separated, the inclination of the inclined surface must be set smaller so that the fed out sheet is not deformed or folded; whereas, when the sheets having small flexural rigidity are separated, the inclination of the inclined surface must be set greater so that the second and other sheets can be fully held by the elastic force of the flexed uppermost sheet.
Accordingly, when the inclination of the inclined surface 207 is set greater to separate the sheets (for example, envelopes, post cards and the like) having great flexural rigidity, for example, when copying sheets having a weight of 60-100 g/m.sup.2 try to be separated, the second and other sheets cannot be adequately held by the elastic force of the flexed uppermost sheet, with the result that the double-feed of sheets may occur. Thus, such an inclined surface cannot be used for the sheet having small flexural rigidity (such as plain sheet).
To avoid this, there has been proposed a technique in which both sheets having different flexural rigidities can be separated by a single separation means. Now, this technique will be explained with reference to FIG. 12. A sheet stacking plate 301 on which sheets are stacked as a sheet stack is biased upwardly by a spring 302 so that, when a sheet supply roller 303 is contacted with an uppermost sheet in the sheet stack, several sheets are fed out by rotation of the sheet supply roller.
An elastically deformable plate member 305 is disposed at a position where tip ends of the stacked sheets is regulated. The plate member 305 is formed from a plastic film or a metallic spring plate having predetermined flexural rigidity and can be elastically deformed when the sheets fed out by the sheet supply roller 303 abuts against the plate member.
When the sheet supply roller 303 is rotated and the sheet stack on the stacking plate 302 is urged against the sheet supply roller 303 by a force of the spring 302 (upon releasing the sheet stacking plate by a release means (not shown)), the plate member is greatly flexed by the tip ends of the driven sheets. In this condition, the tip end of the uppermost sheet rides over the plate member while sliding on the latter, thereby separating the uppermost sheet from the other sheets. With this arrangement, various sheets having different rigidities can be separated.
Incidentally, separation pads 306 for generating a double-feed preventing force by contacting with a last sheet are provided on a sheet stacking surface of the sheet stacking plate 302. The separation pads 306 are normally formed from elastic material such as rubber, artificial leather or the like.
However, in the sheet supply apparatus having the construction shown in FIG. 12, when the sheet supply roller 303 is rotated, the separation pads 306 may be elastically deformed to float. If the separation pads are deformed in this way, as shown in FIG. 13, the tip ends of the sheets are lifted, with the result that the deforming start position of the plate member 305 is elevated. In the separating method using the plate member 305, since the sheets are separated by controlling the elasticity of the plate member 305, if the deforming start position is elevated, an amount of deformation of the plate member 305 is increased, with the result that two or more sheets may ride over the plate member simultaneously, thereby causing the double-feed.
Further, even if the separation pads 306 are formed from material which is hard to be elastically deformed, due to inaccurate parallelism of the sheet stacking surface of the sheet stacking plate 302 and/or dispersion in thickness of the separation pads 306, the above-mentioned phenomenon (floating of the separation pad or pads) may occur, thereby causing the double-feed. Thus, this attempt is still unsatisfactory.